A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes), which in turn vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The scala tympani forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea. In such cases a cochlear implant is an auditory prosthesis which uses an implanted stimulation electrode to bypass the acoustic transducing mechanism of the ear and instead stimulate auditory nerve tissue directly with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processing stage 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant stimulator 108. Besides extracting the audio information, the implant stimulator 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through connected wires 109 to an electrode array 110 inserted into the cochlea. Typically, this electrode array 110 includes multiple electrode contacts on its surface that provide selective stimulation of the cochlea 104. Stimulation is either carried out against an external reference electrode contact (i.e., a remote ground contact) outside the cochlea or against another electrode contact of the array within the cochlea 104.
The insertion of the electrode array 110 requires surgery. Cochlear implant manufacturers offer to surgeons a wide range of mechanical tools necessary for implantation of the device. However, since the electrode array 110 is inserted into the cochlea 104 through a small hole, the surgeon gets no visual conformation of the exact placement of the electrode array 110 within the cochlea 104 itself. Insertion depth may be estimated by the part of the electrode array 110 that has not yet been inserted into the cochlea 104 but apart from that, the exact location of already inserted contacts is unknown during surgery.
Imaging techniques, like X-ray or MRI, may be used to give information about the exact positioning of the electrode array 110. However, all of these methods require additional effort and equipment that is usually not available during surgery. Consequently, the imaging often happens at a later date, when a correction is no longer possible. Instead, electrode contacts will often have to be switched off from stimulation to compensate for a misplacement of the electrode array 110, which may lead to a reduced performance of the cochlear implant.